This invention relates to the production of fiber reinforced resin composite structures, and in particular to processes for vacuum assisted resin transfer molding of large composite structures.
Vacuum assisted resin transfer molding (VA-RTM) has been used to produce a number of large, fiber reinforced composite structures such as boat hulls which incorporate materials such as foam and balsa cores. The cores are covered with a fiber reinforced resin. In the VA-RTM process, the reinforcement fiber, such as a fabric or mat, is arranged in a single sided mold in a dry condition along with the desired core materials according to the form of the desired finished part. The lay-up is then encapsulated in a vacuum bag and impregnated with resin under vacuum. The resin is allowed to cure.
Various methods have been utilized to introduce and enhance the distribution of resin through the reinforcement fiber. These methods include the placement of a disposable distribution media over the outside layer of fabric and the incorporation of holes and/or slots penetrating through the core to allow resin to flow from the outer to the inner layer of reinforcement fiber. See, for example, U.S. Pat. Nos. 5,316,462 and 4,560,523. A supply groove in a foam core has also been used in a closed mold resin injection process to facilitate resin flow. See, for example, U.S. Pat. No. 5,096,651.
The present invention relates to a method for distributing resin during the manufacture of large composite structures using a vacuum assisted resin transfer molding (VA-RTM) process and the composite structure produced by this method. The composite structure is formed from internal cores surrounded by fiber reinforced resin. In one embodiment of the invention, resin is supplied directly into a network of main feeder grooves which are interconnected to a series of smaller microgrooves formed in the surface of the internal cores. From the feeder grooves and microgrooves, the resin flows outwardly from the core to penetrate the reinforcement fiber. In a second embodiment of the invention, a separate distribution medium is interposed between the internal core and the fiber reinforcement. The resin is supplied directly to one or more main feeder grooves in the core surface and penetrates the reinforcement fiber via the distribution medium. Also, the main feeder grooves can extend around the cores to form supply loops, allowing impregnation of transverse structural members.
In a further embodiment, an integrated vacuum bag and mold are formed from a textured sheet of metal. The texture is formed by closely spaced upraised portions on one side of the sheet which correspond with depressions on the other side of the sheet. The closely spaced upraised portions define valleys therebetween which form a resin distribution network. Main feeder grooves are formed directly in the sheet. The textured sheet can also be used as a mold from which other tools are made.
With this method, large composite structures which require multiple cores can be formed quickly prior to the gel time of typical vinyl ester or polyester resins, and the amount of resin used can be minimized. By supplying the resin directly through the vacuum bag into the feeder grooves, the supply is not limited to a part edge or inlet in a tool. Adjacent cores can be supplied via a single resin inlet. The resin distribution network can remain in the finished part, eliminating disposal of distribution materials. In this case, the microgrooves are filled with resin after curing, thereby increasing interlaminar shear strength and delamination strength. Structural features such as shear ties, compression webs, or beams can be incorporated directly into the composite part during the molding process.